1. Field of the Invention
The present invention relates to a sheet comprised of a high density composite material based on discontinuous mineral fibers, e.g., fibers comprised of glass or mineral rock materials.
The term "sheet" as used in connection with the invention encompasses pieces of fixed segment length as well as continuous sheet.
2. Background of the Invention
The discontinuous mineral fibers, particularly glass fibers, are generally employed for thermal insulation and for acoustic insulation. Accordingly, they are present in the form of low density mats or felts, bound together by a polymerized resin. These mats generally contain more than 70 wt. % of glass fibers, with the remainder comprising the polymerized binder; the apparent density of the mat is about 10 kg/m.sup.3.
Non-woven materials based on mineral fibers may also be employed as starting materials for the manufacture of molded panels such as covers for skylights or sunroof openings in automobile roofs, acoustic panels, automobile cowling panels and other miscellaneous body panels, etc. For these applications they are combined with one or more facing sheets under elevated pressure and temperature, to produce the desired configuration of the panel as well as to effect the polymerization of the binder and the cohesion of the constituent elements of said panel. The core produced on the basis of non-woven mineral fiber materials has a mean apparent density after molding which is relatively high but which nonetheless remains generally less than 200 kg/m.sup.3. While more rigid than a felt, this core has very good mechanical properties such as burst strength, modulus of elasticity, and tensile strength.
In a different area of technology, that of glass fiber reinforced plastic materials, continuous glass fibers are generally employed, to serve as reinforcing means. However there is known, from Fr. Pat. No. 2,248,154, a composite material reinforced with glass fibers, employing discontinuous glass fibers. In manufacturing this material, a web or mat of discontinuous fibers is formed to which a liquid or semi-liquid hardenable material is applied, comprised of a synthetic resin, and the fibrous web or mat and the hardenable material are compressed such that the web or mat is impregnated with said material, to obtain a sheet which serves as a mold blank for subsequent molding. This mold blank sheet is comprised of glass fibers in the amount of 30 to 60 wt. %, the remainder being essentially the synthetic resin. The mechanical properties of the product are generally comparable to those of other reinforced plastic materials. However, such reinforced plastic materials are relatively expensive. Furthermore, they are sensitive to temperature variations. In addition, they may deform substantially under load.